There is a great deal of current interest in Li ion conductive solid polymer electrolytes due to their applications in lightweight, high performance, solid-state rechargeable batteries, electrochromic devices, and sensors. When used in a battery they serve two purposes; they are the separator and they are the medium for transporting ions. The classical Li ion conductive solid polymer electrolyte is poly(ethylene oxide) (PEO) complexed with low lattice energy lithium salts such as LiClO.sub.4. These electrolytes, however, exhibit conductivities of 10.sup.-7 to 10.sup.-4 .OMEGA..sup.-1 cm.sup.-1 in the temperature range of 40.degree. to 100.degree. C., which are too low for the fabrication of solid-state batteries for room temperature applications. A room temperature conductivity of about 10.sup.-3 .OMEGA..sup.-1.cm.sup.-1 desired in electrolytes for such batteries.
There has been progress in raising the room temperature conductivities of polymer electrolytes from .ltoreq.10.sup.-7 .OMEGA..sup.-1.cm .sup.-1 to about 10.sup.-5 .OMEGA..sup.-1.cm.sup.-1. This is illustrated by the conductivity of 6.5.times.10.sup.-5 .OMEGA..sup.-1.cm.sup.-1 for the electrolyte derived from poly[bis-((methoxyethoxy)ethoxy)phosphazene] (MEEP) and lithium bis(trifluoromethanesulfone)imide, LiN(CF.sub.3 SO.sub.2).sub.2 (Abraham et at., Chemistry of Materials, 3,339 (1991)). However, a major drawback of MEEP-based electrolytes is their poor mechanical strength. At ambient temperatures, they are glutinous materials and slowly flow under pressure. Consequently, although the MEEP-based electrolytes have 2-3 orders of magnitude higher conductivities than PEO-based electrolytes, they cannot be used as separators in solid-state batteries.
Recently, a group of polymer electrolytes with room temperature conductivities of the order of 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1 have been synthesized in this laboratory (Abraham et al., U.S. Pat. No. 5,219,679). They have been prepared via a non-conventional approach, whereby a polymer matrix such as poly(acrylonitrile) (PAN) is plasticized with solutions of low lattice energy lithium salts in low molecular weight organic solvents with high dielectric constants. The salts include LiN(CF.sub.3 SO.sub.2).sub.2, LiAsF.sub.6, and LiClO.sub.4 and the solvent comprises a mixture of ethylene carbonate (EC) and propylene carbonate (PC). Our work along this line has led to the discovery of other polymer matrices which can be used to prepare polymer electrolytes with high room temperature conductivity. These include poly(vinyl pyrrolidinone) (Abraham et at., U.S. Pat. No. 5,219,679) and poly(vinyl chloride) (Alamgir et at., U.S. Pat. No. 5,252,413).
In this invention, we disclose new solid polymer electrolytes in which the polymer matrix is poly(vinyl sulfone), and they are especially useful for the fabrication of "Li ion" batteries. Lithium ion batteries represent a major advance toward the development of safe, high energy-density, rechargeable batteries. In these batteries, the lithium shuttles between the interstitial sites of the anode and the cathode without the plating of metallic Li. An example of a Li ion battery is the C/LiMn.sub.2 O.sub.4 system in which the anode is graphitic carbon and the cathode is lithiated manganese oxide, LiMn.sub.2 O.sub.4. To effectively function as a separator in such batteries, the polymer electrolyte must be electrochemically stable to the carbon anode, whose potentials lie in the 0.01 to 1.0 V range versus Li.sup.+ /Li, and to the high voltage LiMn.sub.2 O.sub.4 cathode, whose potentials extend up to about 4.5 V versus Li.sup.+ /Li. Consequently, for solid polymer electrolytes to be useful in all-solid-state, rechargeable Li ion batteries with operational capabilities at room temperature, they must have ionic conductivities in the neighborhood of 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1 at room temperature, adequate mechanical stability, and electrochemical stability from 0.0 to about 4.5 V versus Li.sup.+ /Li.